PYROCLASTIC FLOWS AND SURGES OVER WATER - AN EXAMPLE FROM THE 1883 KRAKATAU ERUPTION

Pyroclastic deposits from the 1883 eruption of Krakatau are described from areas northeast of the volcano on the islands of Sebesi, Sebuku, and Lagoendi, and the southeast coast of Sumatra. Massive and poorly s tratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km. Granulometric a nd lithologic characteristics of the deposits indicate that they repre sent the complement of proximal subaerial and submarine pyroclastic fl ow deposits laid down on and close to the Krakatau islands. The distal deposits exhibit a decrease in sorting coefficient, median grain size , and thickness with increasing distance from Krakatau. Crystal fracti onation is consistent with the distal facies being derived from the up per part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic cl asts diminished upwards. The deposits are correlated to a major pyrocl astic flow phase that occurred on the morning of 27 August at approxim ately 10 a.m. Energetic flows spread out away from the volcano at spee ds in excess of 100 km/h and traveled up to 80 km from source. The flo ws retained temperatures high enough to burn victims on the SW coast o f Sumatra. Historical accounts from ships in the Sunda Straits constra in the area affected by the flows to a minimum of 4 x 10(3) km(2). At the distal edge of this area the flows were relatively dilute and turb ulent, yet carried enough material to deposit several tens of centimet ers of tephra. The great mobility of the Krakatau flows from the 10 a. m. activity may be the result of enhanced runout over the sea. It is p roposed that the generation of steam at the flow/sea-water interface m ay have led to a reduction in the sedimentation of particles and conse quently a delay in the time before the flows ceased lateral motion and became buoyantly convective. The buoyant distal edge of these ash- an d steam-laden clouds lifted off into the atmosphere, leading to coolin g, condensation, and mud rain.